ruling question-hidden chips

[otter]

This happened a couple days ago in a $5-10nl game. A player went all in and from what we could see he had $400 in red chips. The other player ($4,000 behind) called pretty quickly). The issue was that behind these 4 stacks were 3 purple chips that you couldn't see unless you were sitting right next to him AND looking for them. Is the caller required to call $1900 or $400?

I'm sure this is pretty basic, but I don't know what the rule is.